Transcript Action potentials only require movement of (relatively) small

Ion Channels
Active Transporters: The proteins that created and
maintain ion gradients
Ion channels : give rise to selective ion permeability
changes
ION CHANNELS
Ion channels are transmembrane proteins that contain a
specialized structure, THE PORE that that allow
particulars ions to cross the membrane.
Some ion channels contain voltage sensor ( voltage
gated channels) that open or close the channel in
response to changes in voltage.
Other gated channels are regulated by extracellular
chemical signals such as neurotransmitter or by
intracellular signals as a second messengers.
ACTIVE TRANSPORTERS
Membrane proteins that produce and maintain ion
concentration gradients.
For example the Na+ pump which utilizes ATP to
regulate internal concentration of Na+ and K+.
Transporters create the ionic gradient that drive
ions through open channels, thus generating
electric signals
What is the mechanism for ion movement
across the membrane?
• K+ and Na+ currents were distinct, suggesting distinct
mechanisms
• Mechanism is voltage dependent (must sense voltage)
• Voltage clamp recordings showed that ions move
across membrane at high rates (~ 600,000 /s) –
inconsistent with an ion pump mechanism
• Ion selectivity of Na+ and K+ currents – size dependent
permeability suggests pore of certain diameter.
• Armstrong (1965-6) – TEA block could be overcome by
adding excess K+ to the extracellular fluid and stepping
to hyperpolarized potentials (K+ comes into cell)
suggesting that K+ ions dislodge TEA from pore
Ion channels share several characteristics
 The flux of ions through the channel is passive .
The kinetic properties of ion permeation are best
described by the channel conductance (g) that is
determinate by measuring the current flux (I) that flows
through the channel in repose to a given electrochemical
driving force. (Electrochemical driving force is determinate
by difference in electric potential across membrane and
gradient of concentration of ions) .
At the single
channel level, the
gating transitions
are stochastic.
They can be
predicted only in
terms of
probability.
Ion channels share several characteristics
 In some channels the current flow varies linearly
with the driving force ( channels behave as resistors)
In other channels, current flow is a non-linear
function of driving force ( Rectifiers)
High conductance (γ)
I (pA)
V (mV)
Low conductance (γ)
Ohmic channel
( I=Vm/R)
Rectifying Channel
Ion channels share several characteristics
 The rate of ion flux (current) depends on the
concentration of the ions in solution ( At low
concentrations the current increases linearly with the
concentration, at higher concentrations the current reach
a saturation point ) .
The ionic concentration at which current flow reaches
half its maximum defines the dissociation constant for
ion binding.
Some ion channels are susceptible to occlusion by free ions
or molecules
The Opening and closing of channels involve
conformational changes
In all channel so far studied, the channel protein has
two or more conformational states that are relatively
stable. Each stable conformation represents a different
functional state.. Each channel has an open state and
one or two closed states. The transition between states is
calling gating.
The Opening and closing of channels involve
conformational changes
Three major regulatory mechanisms have evolved to
control the amount of time that a channel remains open
and active.
 Under the influence of these
regulators ,channels enter one
of three functional states:
closed and activable (resting),
open (active) or closed and
nonactivable ( refractory).
The signal that gate the
channel also controls the rate
of transition between states.
The Opening and closing of channels involve
conformational changes
Ligand -gated and voltage gated channels enter
refractory states through different process. Ligand-gated
channels can enter refractory state when the exposure
to ligand sis prolonged (desensitization)
Voltage-gate channels enter a refractory state after
activation. The process is called inactivation.
Activation is the rapid process
that opens Na+ channels
during a depolarization.
Inactivation is a process that
closes Na+ channels during
depolarization. The membrane
needs to be hyperpolarized for
many milliseconds to remove
inactivation.
The Opening and closing of channels involve
conformational changes
 Exogenous factors such as drugs and toxins can
affect the gating control sites.
Structure of Ion Channels
Ion channels are composed of several subunits. They can
be constructed as heterooligomers from distinct subunits, as
homooligomers from a single type of subunit o from a single
polypeptide chain organized into repeated motifs.
In addition to one or more pore forming unties, which
comprise a central core, some channels contain auxiliary
subunits which modulate the characteristics of the central
core
Structure of voltage
gated ion channels
Repeated series of 6 TM a helices
S4 helix is voltage sensor
Loop between S5 & S6 composes
selectivity filter
Gating currents
Movement of + charges in S4
segment produces small
outward current that
precedes ion flux through
channel
Role of auxiliary subunits
Auxiliary (non pore)
subunits affect:
• Surface expression
• Gating properties
Voltage gated sodium
channels
A large alpha subunit that
forms the core of the channel
and its functional on its own. It
can associate with beta
subunits
Blocked by: TTX, STX, *cain
local anesthetics
Persistent (noninactivating) Na+
currents are
produced by an
alternative
channel gating
mode
Functions of voltage-gated Na+ channel alpha subunits
Protein name
Gene
Expression profile
Associated
channelopathies
NaV 1.1
SCN1A
Central and peripheral
neurons and cardiac
myocites
Febrile epilepsy,
severe myclonic
epilepsy of infancy,
infantile spasms,
intractable childhood
epilepsy, familial
autisms
Nav1.2
SCN2A
Central and peripheral
neurons
Febrile seizures and
epilepsy
Nav1.4
SCN4A
Skeletal muscle
Periodic paralysis,
potassium agravated
myotonia
Nav1.5
SCN5A
Cardiac myocites,
skeletal muscle,
central neurons
Idiopathic ventricular
fibrillation
Nav1.7
SCN9A
Dorsal root ganglia,
peripheral neurons.
Heart, glia
Insensitivity to pain.
Voltage gated Ca2+ channels
Gene Product
Tsien Type
Characteristics
Cav1.1-1.4
Cav2.1
“L”
“P/Q”
High voltage
Mod voltage
activated,
activated,
slow inactivation moderate
(Ca2+ dependent) inactivation
Cav2.2
“N”
High voltage
activated,
moderate
inactivation
Cav2.3
“R”
Mod voltage
activated
fast
inactivation
Cav3.1-3.3
“T”
Low voltage
activated
fast
inactivation
Blocked by
dihydropiridines Agatoxin
Conotoxin
SNX 482
Mibefridil
(nimodipine)
GVIA
High Ni2+
IVA
Form by different subunits:α1, α2δ,β and γ. The α1 subunit
forms the pore, the other subunits modulate gating.
Ca2+ dependent
Ca2+ channel
inactivation
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
channel
CaM
Ca2+
-
Ca2+
Ca2+
Ca2+
Potassium Channels
Voltage gated
Inwardly
rectifying
2 pore (“leak”)
Ca2+
activated
Inwardly-rectifying and “leak” K+
channels
Inwardly-rectifying
Inwardly-rectifying channels
• a subunits: Kir 1.X - 7.X
• Rectifying character due to internal block by
Mg2+ and polyamines
• Roles:
• Constitutively active resting K+
conductance (eg. Kir1, Kir2)
• G-protein activated (Kir3)
• ATP sensitive (Kir6)
2 pore “leak” channels
• many different a subunits, nomenclature
still argued
• Outwardly rectifying due to unequal [K+]
across the membrane
• Roles:
• Constitutively active resting K+
conductance
• pH sensing
• Mechanosensitive
• Thermosensitive
• Second messenger sensitive (cAMP, PKC,
arachadonic acid)
2 pore “leak”
Voltage gated K+ channels
Gene Product
Kv1.X (1-8)
“D type”
Kv2.X (1-2)
“Delayed
rectifier”
Kv3.X (1-4)
“Delayed
rectifier”
Kv4.X (1-4)
“A type”
Kv7.X (1-5)
“M current”
Characteristics
Low voltage
activated (~50 mV),
fast activation
(< 10 ms)
slow inactivation
High voltage
activated (0 mV),
mod activation
(>20 ms)
very slow
inactivation
High voltage
activated (-10 mV),
fast activation
(10-20 ms)
very slow
inactivation
fast deactivation
Low voltage
activated (-60 mV)
fast activation
(10-20 ms)
fast
inactivation
Low voltage
activated (-60 mV)
slow activation
(>100 ms)
no
inactivation
Blocked by
4-AP (100 µM)
TEA (5-10 mM)
4AP (1-5 mM)
TEA (0.1-0.5 mM) 4-AP (5 mM)
4AP (0.5-1 mM)
BDS (50 nM)
dendrotoxin
Kv4
(“A
type”)
Kv1
(“D
type”)
Kv2
(“DR
type”)
Kv3
(“DR
type”)
XE991
Ca2+ activated K+ channels - role in repolarization
following APs
Spike frequency accommodation
Voltage response
currents mediating AHP
Role of IKCa in burst duration
Ca2+ activated K+ channels
Channel Type
BK
SK
sAHP
“maxi K, IC fAHP” “mAHP”
“sAHP”
Gene product
slo 1-3
SK1-3
????
Voltage dep?
Yes
No
No
[Ca2+] to activate 1-10 µM
0.1-1 µM
0.1-1 µM
Ca2+ binding
direct to
asubunit
calmodulin
hippocalcin?
Single channel 100-400 pS
5-20 pS
5-10 pS
Conductance
Blocked by
charybdotoxin
apamin
TEA (> 20
mM)
TEA (< 1 mM)
TEA (> 20 mM)
Many drugs and toxins act on
voltage gated ion channels
Effect of drugs and toxins
• Many toxins block ion channels
directly either from the outer
(TTX) or inner (lidocaine)
surface of the channel
• Other toxins change the
properties of the channel
without blocking it
– Delaying inactivation
– Shifting voltage dependence
TTX
LA
FUGU
Modulation of Ion Channels
Example, enhancement of
Ca2+ channels in cardiac
myocytes by NE
Dendritic ion channels participate in
synaptic amplification and
integration
Channelopathies
Condition
Channel type
Paramyotonia congenita
Vgated Na+ channel
Hemiplegia of childhood
Na+/K+ ATPase
Congenital hyperinsulinism
IR K+ channel
Cystic fibrosis
Cl- Channel
Episodic ataxia
Vgated K+ channel
Erythromegalia
Vgated K+ channel
Generalized epilepsy with febrile
seisures
Vgated Na+ channel
Hyperkalemic periodic paralysis
Vgated Na+ channel
Malignant hyperthermia
L gated Ca2+ channel
Myasthenia Gravis
Lgated Na+ channel
Neuromyotonia
Vgated K+ channel
Recommended Readings:
Kandel. Principles of Neural Science, 4 th Edition chapter: 6
Hille. Ion Channels of Excitable Membranes. 3 ed. Edition.